Re: Another view on analysis and ER

From: David BL <davidbl_at_iinet.net.au>
Date: Wed, 5 Dec 2007 20:33:59 -0800 (PST)
Message-ID: <ae460150-e03d-4b64-bfe3-f4c6ac52912a_at_e10g2000prf.googlegroups.com>


On Dec 6, 10:47 am, JOG <j..._at_cs.nott.ac.uk> wrote:

> Either we are crossing terminology, or this has already been
> highlighted earlier in the thread with reference to marriages. E/R
> forces one to pick a single conceptual viewpoint (marriage as
> relationship/marriage as entity, etc), whereas a propositional
> encoding is neutral on the topic.

There is something I may be misunderstanding - can you put me right?

I think an entity is characterised as something in the real world that we want to be able to reference using a reasonably small identifier. Typically an entity is subject to change over time, and we may not have convenient access to a nice, convenient, small number of measurable attributes that uniquely identifies it. Nevertheless we believe that the identifier can be used successfully in the real world. This could depend on the population statistics, such as the way we identify people around us by their facial characteristics, and it is helpful that people don't tend to have extensive plastic surgery, exchange body parts on a regular basis or cross dress. This allows them to be identified by names like "John".

By contrast I think a relationship is characterised as only being identified by the entities that it relates.

Now I see the same distinction being made in a propositional encoding.

The intensional definition of the following predicate implicitly assumes Husband, Wife and Location correspond to entity types that can be identified using domain values. Evidently marriage is merely a relationship between them.

    married(Husband, Wife, Location) :- Husband married Wife at Location

By contrast the following predicates are consistent with thinking of a marriage itself as an entity

    husband(MarriageId, Husband).
    wife(MarriageId, Wife).
    location(MarriageId, Location).

or maybe just

    married(MarriageId, Husband, Wife, Location)

Now whether one "thinks in ER" or "thinks in propositional encodings", there has to be good reason to introduce a MarriageId.

Aren't you implying that a propositional encoding doesn't commit you to a decision about whether a marriage is implicitly or explicitly identified? I fail to see how that is possible. Received on Thu Dec 06 2007 - 05:33:59 CET

Original text of this message